In recent years the application of liquid crystal display devices has rapidly spread into not only information-communication devices, but general electrical equipment as well. Since liquid crystal display devices do not themselves emit light, transmissive-type liquid crystal display devices provided with a backlight are commonly employed. However, because the backlight consumes a large amount of power, particularly for portable devices reflective-type liquid crystal display devices which do not require a backlight are being used to reduce power consumption. Nonetheless, since reflective liquid crystal display devices utilize external light as a light source, it is difficult to view these devices in a dark room or similar environment. In view of this, in recent years development has been progressing into semi-transmissive-type liquid crystal display devices which especially combine transmissive-type and reflective-type qualities (disclosed in Japanese Patent Laid-Open No. 11-101992 and Japanese Patent Laid-Open No. 2005-106997).
Such a semi-transmissive liquid crystal display device comprises, in a single pixel region, a transmissive part comprising a pixel electrode, and a reflective part comprising both a pixel electrode and a reflective electrode. In dark places, the backlight is lit up and the pixel region transmissive part is utilized to display an image, while in bright places, external light is utilized at the reflective part to display an image without the backlight being lit up. This means that the backlight does not have to be lit at all times, which has the advantage that power consumption can be dramatically reduced.
One example of such a semi-transmissive liquid crystal display device will be explained with reference to FIGS. 3 and 4. FIG. 3 is a plan view of one pixel displayed when viewed through the color filter substrate of a conventional semi-transmissive liquid crystal display device. FIG. 4A is a cross-sectional view along the line IVA-IVA of FIG. 3, and FIG. 4B is a cross-sectional view along the line IVB-IVB of FIG. 3.
This semi-transmissive liquid crystal display device 10A is formed, directly through a plurality of scanning lines 12 and signal lines 13 or via an inorganic insulating film 14, into a matrix pattern on a transparent insulating glass substrate 11. Here, the region enclosed by the scanning lines 12 and signal lines 13 corresponds to one pixel, wherein a TFT (Thin Film Transistor) (not shown) acting as a switching device is formed for each pixel.
Further, an interlayer film 17 made of an organic insulating film is laminated so as to cover the scanning lines 12, signal lines 13 and inorganic insulating film 14, wherein tiny uneven portions are formed on the surface at the reflective part 15 and wherein the surface at the transmissive part 16 is made flat. It is noted that in FIGS. 3 and 4 the uneven portions of the reflective part 15 have been omitted. The interlayer film 17 is provided with a contact hole 20 at a location corresponding to the drain electrode of the TFT. For each pixel, a reflective electrode 18 made of aluminum metal, for example, on its reflective part 15 is formed on the contact hole 20 and the surface of the interlayer film 17, and a transparent pixel electrode 19 made of ITO (Indium Tin Oxide), for example, is formed on the surface of this reflective electrode 18 and the surface of the interlayer film 17 of the transmissive part 16.
In this semi-transmissive liquid crystal display device 10A, the widths of the scanning lines 12 and signal lines 13 are all designed to be formed identically, so that the width L1 of the signal lines 13 in the transmissive part 16 is the same as the width L3 of the signal lines in the reflective part 15. In addition, at the transmissive part 16, the pixel electrode 19 is formed so as not to be in contact with the pixel electrode or the reflective electrode of the pixels adjacent thereto, and, so as to slightly overlap with a scanning line 12 and signal line 13. Among these, the pixel electrode 19 and a signal line 13 are formed so that the width L2 only is overlapping in order to prevent light leakage. Moreover, at the reflective part 15, the reflective electrode 18 and the pixel electrode 19 are formed so as not to be in contact with the reflective electrode or the pixel electrode of the pixels adjacent thereto, and, so as to slightly overlap with a scanning line 12 and signal line 13 to similarly prevent light leakage. Among these, the overlapping width L4 between the pixel electrode 19 and a signal line 13 is formed to be essentially the same as that of said L2.
In addition, a backlight device is provided at a lower portion of the glass substrate 11 comprising (not shown) a well-known light source, light guide plate, diffusion sheet and the like. Further, an oriented film (not shown) is laminated on the surface of the pixel electrode 19 so as to cover all the pixels. A color filter substrate (not shown) provided with elements such as an R, G, B three-color color filter formed to correspond to each pixel and opposing electrodes, faces this glass substrate 11. A sealant is provided surrounding both the substrates, whereby the two substrates are stuck together. The semi-transmissive liquid crystal display device 10A is formed by injecting liquid crystal in between the two substrates.
Thus, in the conventional semi-transmissive liquid crystal display device 10A, by forming so that the reflective electrode 18 and pixel electrode 19 slightly overlap a scanning line 12 and signal line 13, the light leakage from this portion can be prevented, thereby achieving an improvement in contrast.
However, as illustrated in FIGS. 4a and 4b, if a pixel electrode is formed on the scanning line and signal line, a given capacitance Csd is present between the scanning line 12 and signal line 13 and the pixel electrode 19 formed on the reflective electrode 18. If this capacitance Csd equals or goes above a certain value, flicker and cross-talk will occur when activating the liquid crystal display device. Moreover, the mechanisms respectively causing flicker and cross-talk are different. Namely, because the pixel electrodes are activated in an alternating manner with respect to their opposing electrode, the polarity of the voltage applied to the pixel electrode alternately switches with respect to the opposing electrode voltage according to a given cycle (e.g. 60 Hz). However, flicker occurs when the pixel electrode voltage changes, occurring at the side further away from the drive terminal of the scanning line, and not at the side closer to the drive terminal.
In contrast, particularly when black is displayed on a white background screen, cross-talk occurs in the vicinity of the black being displayed. The mechanism for such cross-talk occurrence is thought to be caused as a result of the following. That is, FIG. 5 illustrates a screen on which cross-talk has occurred at the semi-transmissive liquid crystal display device 10A illustrated in FIGS. 3 and 4, for example. When a black screen is displayed on a white background as illustrated in FIG. 5, if the white background region is represented by point X and above and below the black screen, i.e. the region on the signal line side, is represented by point Y, the voltage waveform at the respective points X, Y are as illustrated in FIG. 6.
FIG. 6 illustrates that if a signal is applied to the gate electrode of a TFT, the TFT is activated to initiate writing on the pixel electrode. The potential of the pixel electrode at this point is maintained for a prescribed period of time by an auxiliary electrode capacitance (refer to FIG. 6A). The potential used for white display which is write onto the pixel electrode in this period fluctuates during the retention period in accordance with the amplitude of the opposing electrode potential Vcom (refer to FIG. 6B). In this state, if the voltage waveform that is being applied to the signal line and pixel electrode at points X and Y is observed, at the signal line of the point X portion, a white display voltage is being constantly applied until the next write period arrives, so that the potential of the pixel electrode of this point X portion fluctuates by the same amplitude until the next write period arrives (refer to FIGS. 6C and 6D).
On the other hand, if en route the voltage used for black display is applied to the signal line of the point Y portion, the amplitude of the pixel electrode potential of this point Y portion varies during the period that the voltage used for black display is being applied to the signal line (refer to FIG. 6E). As a result, the actual value of the voltage applied to the liquid crystals differs between points X and Y, whereby a delta ΔV is generated. This delta is manifested as a difference in brightness, thus becoming a cause of cross-talk occurrence (refer to FIG. 6F).
This delta ΔV which is a cause of cross-talk can be expressed by the below equation 1 in accordance with a single pixel equivalent circuit of the liquid crystal display device illustrated in FIG. 7.ΔV=Csd×Vcom÷(C1c+Cst+Csd)  (1)wherein,    Cst: auxiliary electrode capacitance    Csd: capacitance between source/drain    C1c: liquid crystal capacitance
Therefore, it can be seen from this equation 1 that the smaller the capacitance Csd between source/drain is, the smaller the delta ΔV becomes.
Thus, lowering the capacitance Csd between source/drain, i.e. making the overlap between a signal line and the pixel electrode smaller, results in a smaller delta ΔV, whereby cross-talk can be reduced.
However, the major cause of light leakage lies in the disarray of the orientation of the liquid crystals at the end of the pixel electrode. Thus, if the overlap between the signal line and the pixel electrode is made smaller in order to reduce cross-talk, the problem arises that light leakage increases, especially at the transmissive part, thereby causing contrast to decrease.